1. Field of the Invention
The present invention relates to a method of and an apparatus for adjusting the weight of a workpiece to a desired value by machining a weight-adjustment portion formed on the workpiece.
2. Description of the Prior Art
It has been desired to reduce the vibration of automobile engine during operation. For this, it is necessary to ensure that such moving parts as the pistons, connecting rods, and crankshaft are of accurately controlled weight so that the crankshaft can rotate in a well balanced condition. Above all, the connecting rods, which convert reciprocating movement of the piston into rotating movement of the crankshaft, have to be accurately adjusted in weight because they have a particularly large effect on whether the crankshaft rotates in a well balanced condition.
A known method of balancing rotating members is disclosed in Japanese Patent Public-Disclosure No. 59-65660 in which a ring gear for starting an engine is fixed around the body of a torque converter by MIG welding in such manner that the weight of each weld bead is adjusted so as to obtain balanced rotation of the ring gear. Such method can only be utilized when welding is used. Further the method is not useful where the differences in weight among workpiece is rather large, as in the case of casted connecting rods.
Thus, it might be considered that for a connecting rod, a useful method would be to form a portion for weight adjustment on both ends of the connecting rod and then to machine these portions with an appropriate cutting tool to such degree as required to match the weight of the connecting rod with a predetermined weight. In this case, since the difference in weight among casted connecting rods is large it is desirable to use two-stage machining process. In this process, the cutting tool is first positioned at a predetermined reference feeding position and driven to machine the weight-adjustment portion. After the first machining is finished the cutting tool is moved to a predetermined second feeding position and is driven again to machine the same portion. Thus by the first and the second machining the weight of the connecting rod is adjusted to the desired value. Further two cutting tools, one for each machining operation, are preferably used for the purpose of improving operational efficiency where large numbers of connecting rods are processed.
However, the bit of the cutting tool will wear over long-term operation and, therefore, the cutting amount gradually becomes less even if the cutting tool is positioned precisely. Thus, it is impossible to perform an accurate machining operation for obtaining a connecting rod with a desired weight. The same problem occurs when two cutting tool are used as mentioned above. In this case, the wear rates are different between the two cutting tool, so that it is even more difficult to obtain accurate machining.